ACTG 362-A Randomized, Placebo Controlled Trial to Determine the Efficacy of Azithromycin Prophylaxis for the Primary Prevention of Mycobacterium Avium Complex (MAC) Disease and Other Infections in Subjects Who Have Achieved and Increase in CD4 Cells on Antiretroviral Therapy. The advent of highly active antiretroviral chemotherapy has provided an opportunity for much more substantial and prolonged improvements in cellular and humoral immunity. This study is designed to test the hypothesis that HIV- infected patients who previously have had a CD4 count of <50 cells/mm3 and who have had a sustained increase in CD4 count to >100 cells/mm3 in response to antiretroviral therapy are no longer at sufficiently increased risk of developing MAC disease to require MAC prophylaxis, i.e., that it is "safe" to discontinue or not initiate MAC prophylaxis for such patients. In addition, this cohort will provide an opportunity to examine cellular immune function and determine whether simple measures of immune function taken at baseline and during follow-up can predict which subjects are at continued risk for opportunistic infections despite the expansion in CD4 cell number in response to antiretroviral therapy. The study will also help further define the relationship between viral load and risk for MAC disease and other opportunistic infections. This NIH- sponsored, investigator-initiated 'A' study is a randomized, double-blind, placebo-controlled trial which will be available for enrollment to any subject who is receiving antiretroviral therapy and has had a documented increase in CD4 cell count from below 50/mm3 on one occasion to above 100 cells/mm3 on two separate occasions, sequentially, at least 4 weeks apart (the last test being performed at an ACTG-approved laboratory within 14 days of study entry). There will be 636 subjects enrolled (318 in each arm) at multiple centers. Subjects will be randomized to receive azithromycin 1200 mg once weekly or matching placebo and will be followed every 8 weeks until study closure or for 18 months (72 weeks) after the last subject is enrolled. Subjects who develop a drop in CD4 count below 50 cells/mm3 on two measurements at least 4 weeks apart will be offered open-label azithromycin. Subjects will be stratified at baseline for prior use of MAC prophylaxis into three groups; no prophylaxis, prior azithromycin prophylaxis and other MAC prophylaxis.